Synthesis and Magnetic Characterization of Zinc Ferrite Nanoparticles

Rennes 1, CS 74205, 35042 Rennes CEDEX, France, and Institut de Chimie de la Matie`re ... UMR 6511, Institut de Chimie de Rennes, Université de Renne...
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Langmuir 2002, 18, 8209-8216

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Synthesis and Magnetic Characterization of Zinc Ferrite Nanoparticles with Different Environments: Powder, Colloidal Solution, and Zinc Ferrite-Silica Core-Shell Nanoparticles F. Grasset,*,†,‡ N. Labhsetwar,† D. Li,† D. C. Park,† N. Saito,† H. Haneda,† O. Cador,§ T. Roisnel,| S. Mornet,⊥ E. Duguet,⊥ J. Portier,⊥ and J. Etourneau⊥ National Institute for Materials Science-Advanced Materials Laboratory, NIMS-AML, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan, Laboratoire Verres et Ce´ ramiques, UMR 6512, Institut de Chimie de Rennes, Universite´ de Rennes 1, CS 74205, 35042 Rennes CEDEX, France, Magnetic Materials Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1, Hirosawa, Wako, Saitama, 351-0198, Japan, Laboratoire de Chimie du Solide et Inorganique Mole´ culaire, UMR 6511, Institut de Chimie de Rennes, Universite´ de Rennes 1, CS 74205, 35042 Rennes CEDEX, France, and Institut de Chimie de la Matie` re Condense´ e de Bordeaux, ICMCB-CNRS, Universite´ Bordeaux-1, 87 avenue du Dr Albert Schweitzer, F-33608 Pessac, France Received April 4, 2002. In Final Form: June 21, 2002 Synthesis of nanoparticles under restricted environments offered by water-in-oil microemulsions provides excellent control over particle size and shape and interparticle spacing. These environments have been used in the synthesis of silica nanoparticles with a ZnFe2O4 magnetic core. First, aqueous magnetic fluids constituted of zinc ferrite nanoparticles with a size ranging between 4 and 6 nm have been synthesized using a soft chemical approach. Chemical analysis has shown that the zinc ferrite nanoparticles are nonstoichiometric with the estimated formula Zn0.87Fe2.09X0.04O4 (X represents vacancies). The obtained silica nanoparticles (40-60 nm) with a zinc ferrite magnetic core (4-6 nm) have been characterized by X-ray diffraction, electron microscopy, and magnetization measurements. Preliminary magnetic measurements have inferred that the magnetic properties of these nanoparticles at low temperature are essentially governed by the interface particle-habitat.

1. Introduction Magnetic properties of nanoparticles have recently attracted considerable attention from both scientific and technological points of view.1,2 Particularly, ZnFe2O4 nanoparticles have generated a large research effort in the past 10 years because their magnetic properties differ markedly from those of their bulk counterpart.3-23 Bulk * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +33 (0)2 23 23 65 40. Fax: +33 (0)2 23 23 56 83. † National Institute for Materials Science-Advanced Materials Laboratory. ‡ Laboratoire Verres et Ce ´ ramiques, UMR 6512, Institut de Chimie de Rennes, Universite´ de Rennes 1. § Magnetic Materials Laboratory, RIKEN (The Institute of Physical and Chemical Research). | Laboratoire de Chimie du Solide et Inorganique Mole ´ culaire, UMR 6511, Institut de Chimie de Rennes, Universite´ de Rennes 1. ⊥ Institut de Chimie de la Matie ` re Condense´e de Bordeaux, ICMCB-CNRS, Universite´ Bordeaux-1. (1) Dormann, J.; Fiorani, D.; Tronc, E. Magnetic relaxation in FineParticle Systems. In Advances in Chemical Physics, Vol. 98; Prigogine, I., Rice, S. A., Eds.; John Wiley and Sons: New York, 1997; pp 283-494. (2) Ferrites: Proceedings of the Eighth International Conference on Ferrites, ICF8, Kyoto 2000; Published by the Japan Society of Powder and Powder Metallurgy, 2001. (3) Goya, G.; Rechenberg, H.; Chen, M.; Yelon, W. J. Appl. Phys. 2000, 87, 8005-8007. (4) Yokoyama, M.; Oku, T.; Taniyama, T.; Sato, T.; Ohta, E.; Sato, T.; Haneda, K.; Itoh, S.; Kurahashi, K.; Takeda, M. Physica B 1995, 213-214, 251-253. (5) Hochepied, J.; Bonville, P.; Pileni, M. J. Phys. Chem. B 2000, 104, 905-912. (6) Burghart, F.; Potzel, W.; Kalvius, G.; Schreier, E.; Grosse, G.; Noakes, D.; Schafer, W.; Kockelmann, W.; Campbell, S.; Kaczmarek, W.; Martin, A.; Krause, M. Physica B 2000, 289-290, 286-290.

ZnFe2O4 is a normal spinel form with δ ) 1 (corresponding to the formula (AδB1-δ)[A1-δBδ]2O4, where δ is the inversion parameter, round and square brackets denote the tetrahedral (A-sites) and octahedral (B-sites) sites, respectively), the diamagnetic Zn2+ ions occupying only A-sites. (7) Goya, G.; Rechenberg, H. J. Magn. Magn. Mater. 1999, 196-197, 191-192. (8) Chinnasamy, C.; Narayanasamy, A.; Ponpandian, N.; Chattopadhyay, K.; Guerault, H.; Greneche, J. J. Phys.: Condens. Matter 2000, 12, 7795-7805. (9) Sepelak, V.; Steinike, U.; Uecker, D.; Wibmann, S.; Becker, K. J. Solid State Chem. 1998, 135, 52-58. (10) Andres-Verges, M.; Martinez, M.; Matijevic, E. J. Mater. Res. 1993, 8, 2916-2920. (11) Goya, G.; Rechenberg, H. J. Magn. Magn. Mater. 1999, 203, 141-142. (12) Hamdeh, H.; Ho, J.; Oliver, S.; Willey, R.; Oliveri, G.; Busca, G. J. Appl. Phys. 1997, 81, 1851-1857. (13) Oliver, S.; Harris, V.; Hamdeh, H.; Ho, J. Appl. Phys. Lett. 2000, 76, 2761-2763. (14) Ho, J.; Hamdeh, H.; Chen, Y.; Lin, S.; Yao, Y.; Willey, R.; Oliver, S. Phys. Rev. B 1995, 52, 10122-10126. (15) Oliver, S.; Hamdeh, H.; Ho, J. Phys. Rev. B 1999, 60, 34003405. (16) Hamdeh, H.; Ho, J.; Oliver, S.; Willey, R.; Kramer, J.; Chen, Y.; Lin, S.; Yao, Y.; Daturi, M.; Busca, G. IEEE Trans. Magn. 1995, 31, 3808-3810. (17) Jeysdevan, B.; Tohi, K.; Nakatsuka, K. J. Appl. Phys. 1994, 76, 6325-6327. (18) Zhihao, Y.; Lide, Z. Mater. Res. Bull. 1998, 33, 1587-1592. (19) Morais, P.; Da Silva, S.; Soler, M.; Sousa, M.; Tourinho, F. J. Magn. Magn. Mater. 1999, 201, 105-109. (20) Anantharaman, M.; Jagathesan, S.; Malini, K.; Sindhu, S.; Narayanasamy, A.; Chinnasamy, C.; Jacobs, J.; Reijne, S.; Seshan, K.; Smits, R.; Brongerma, H. J. Magn. Magn. Mater. 1998, 189, 83-88. (21) Clark, T.; Eveans, B. IEEE Trans. Magn. 1997, 33, 3745-3747. (22) Andres-Verges, M.; de Julian, C.; Gonzalez, J.; Serna, C. J. J. Mater. Sci. 1993, 28, 2962-2966. (23) Kamiyama, T.; Haneda, K.; Sato, T.; Ikeda, S.; Asano, H. Solid State Commun. 1992, 81, 563-566.

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Consequently, all the Fe3+ are in B-sites and are coupled between each other via a superexchange pathway through A-sites. The B-B interactions being very weak, the normal spinel ZnFe2O4 shows long-range antiferromagnetic ordering at TN ≈ 9-11 K.6,14,24 Recently, the coexistence of both short- and long-range order has been demonstrated,6,24 but the physical origin of the establishment of the short-range order is still under debate. In the case of the nanoparticles, the situation is more complicated and confused because of the inversion of the cation distribution, size effects, and nonstoichiometry. If the noninverted stoichiometric nanoparticles have an antiferromagnetic ground state (TN ) 13 K)25 like the bulk material, the ground state of nanosized inverted zinc ferrites is magnetic with a large magnetization which is generally explained by the distribution of the cations.3-5,7,8,11-17,20,23 Indeed, in the partially inverted stoichiometric spinel, some Zn2+ ions occupy B-sites while some Fe3+ ions are in the tetrahedral ones leading to a magnetically active A sublattice which strongly interacts with the B sublattice. Values of δ ranging from 0.03 to 0.6 are reported in the literature, depending strongly upon the preparation procedures.3,12-16 ZnFe2O4 nanoparticles have been synthesized with a large variety of methods: coprecipitation,4,17,19-21,23,26,27,33 microemulsion,5,18 supercritical sol-gel processing,12-16 hydrothermal synthesis,10,25 or high-energy ball milling.3,7-9,11 The magnetization has been found to increase with grain size reduction. This feature is generally associated with the increase of the cation inversion and with the diminution of the size of the grains.5,8,20 The size polydispersity and the mutual interaction between magnetic particles complicate the interpretation of the magnetic properties of powder nanoparticles.1,28 Modifying the properties of one material by coating it with another type of material has been a popular approach, for which it is impossible to give an exhaustive list of references. In the case of coating magnetic oxide particles with silica, various routes have been investigated: aggregation of small colloids,29 condensation of silica oligomers produced by solubilization of silica particles in a highly basic medium,30 hydrolysis of silicon alkoxides,31 and the microemulsion route.32 But with the first three processes, it is very difficult to obtain well-designed silica nanoparticles in the range from 5 to 100 nm. Therefore, in this study the water-in-oil (W/O) microemulsion route was chosen because it provides an unique environment to synthesize novel inorganic magnetic materials with interesting designs and/or specific properties,33,34 resulting in nonaggregated nanoparticles. Nevertheless, at the present time, one of the major problems with the micro(24) Schiessl, W.; Potzel, W.; Karzel, H.; Steiner, M.; Kalvius, G.; Martin, A.; Krause, M.; Halevy, I.; Gal, J.; Schafer, W.; Will, G.; Hillberg, M.; Wappling, R. Phys. Rev. B 1996, 53, 9143-9152. (25) Pannaparayil, T.; Komarnen, S.; Marande, R.; Zadarko, M. J. Appl. Phys. 1990, 67, 5509-5511. (26) Zins, D.; Cabuil, V.; Massart, R. J. Mol. Liq. 1999, 83, 217-232. (27) Sato, T.; Haneda, K.; Seki, M.; Iijima, T. Appl. Phys. 1990, A50, 13-16. (28) Garcia-Otero, J.; Porto, M.; Rivas, J.; Bunde, A. Phys. Rev. Lett. 2000, 84, 167-170. (29) Homola, A. P.; Lorentz, M. R.; Suusner, H.; Rice, S. J. Appl. Phys. 1987, 61, 3898. (30) Philips, A. P.; Van Brugge, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (31) Klotz, M.; Ayral, A.; Guizard, C.; Me´nager, C.; Cabuil, V. J. Colloid Interface Sci. 1999, 220, 357-361. (32) Handbook of microemulsion science and technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, 1999. (33) Tago, T.; Nagase, R.; Hatsuta, T.; Kishida, M.; Wakabayshi, K. Ferrites: Proceedings of the Eighth International Conference on Ferrites, ICF8, Kyoto 2000; Published by the Japan Society of Powder and Powder Metallurgy, 2001; pp 763-765.

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emulsion process remains the effect of the reactants and products on the stability domain of the microemulsion, particularly the metals concentration in the aqueous pseudophase used for precipitation reactions.32 Avoiding this classical precipitation process, we propose in our paper an original method to synthesize designed nanoparticles using a colloidal suspension (ferrofluid). This colloidal suspension, used as a starting component of the aqueous pseudophase, allows one to increase the metal concentration without destabilizing the microemulsion. The aim of this present work was to obtain more information about chemical, structural, and magnetic properties of ZnFe2O4-SiO2 core-shell nanoparticles prepared by the method aforementioned. A low-temperature coprecipitation route and a ferrofluid technology were first used to prepare a colloidal suspension of zinc ferrite nanoparticles. The obtained zinc ferrite (powder and colloidal solution) and silica nanoparticles with a magnetic zinc ferrite core have been characterized by X-ray diffraction, electron microscopy, and specific surface area measurements. The preliminary magnetic results are also presented and discussed. 2. Experimental Section 2.1. Products. All the chemical reagents were of 99% (minimum) purity and used without further purification: Fe(NO3)3‚9H2O, Zn(NO3)2‚6H2O, acetone, ethanol, n-heptane, npropanol, NH4OH solution (28%), NaOH pellets, and sodium dodecyl sulfate (SDS) were provided by Wako Chemical Co., and tetraethoxysilane (TEOS) by Shinetsu Japan. The bis[2-ethylhexyl] sulfosuccinate, sodium salt (AOT), and the polyoxyethylene(4) lauryl ether (Brij30) were provided by Sigma Chemical Co. The AOT and Brij30 surfactants were selected for this study because they are readily soluble in saturated hydrocarbons such as n-heptane at room temperature. 2.2. Synthesis of the Colloidal Aqueous Solution of Zinc Ferrite Nanoparticles. An aqueous colloidal suspension was prepared according to the ionic ferrofluid process developed by Massart,26 with few modifications. In our case, ferrite nanoparticles were first obtained by alkalizing, with boiled NaOH, an aqueous mixture of Zn(NO3)2 and Fe(NO3)3 (concentration, 1 mol/ L; Zn2+/Fe3+ ) 0.5). Contrary to nanoparticles of Fe3O4 which are synthesized by coprecipitation in an alkaline medium of Fe3+ and Fe2+ (ratio Fe2+/Fe3+ ) 0.5) at room temperature, the MFe2O4 (M2+ ) Cu, Ni, Zn) ferrite nanoparticles must be synthesized by coprecipitation of M2+ and Fe3+ cations at 100 °C.23,35 Moreover, after coprecipitation at 100 °C, the solution has to be brought to a boil for 90 min. As already observed,23 the temperature significantly accelerates the formation of ferrites and during the boiling time, dissolution-crystallization processes could take place. It is necessary to choose the starting Zn2+-Fe3+ mixture acidity and composition in order to avoid hydroxide precipitation or the formation of R-FeOOH. After precipitation, as found for ferrite by others,23,26,36,37 a sufficient electrostatic repulsion between particles is needed to obtain a stable aqueous colloidal dispersion of zinc ferrite. The particles were dispersed in nitric acid (15 min; concentration, 2 mol/L) under vigorous stirring in order to create positive surface charges (peptization). The acidic precipitate was isolated by decantation on a magnet and/or centrifugation (5000 rpm, 15 min), washed in acetone, and dispersed in pure water. The pH value of the ferrofluid was ≈2. From the single batch of fresh ferrofluid, the totality was flocculated with acetone, followed by separation in a centrifuge at 5000 rpm for 15 min. Subsequently, one part was dried at (34) Mornet, S.; Grasset, F.; Duguet, E.; Portier, J. Ferrites: Proceedings of the Eighth International Conference on Ferrites, ICF8, Kyoto 2000; Published by the Japan Society of Powder and Powder Metallurgy, 2001; pp 766-768. (35) Sousa, M.; Tourinho, F.; Depeyrot, J.; Da Silva, G.; Lara, M. J. Phys. Chem. B 2001, 105, 1168-1175. (36) Arriagada, F. J.; Osseo-Asare, K. Colloids Surf., A 1999, 154, 311-326. (37) Audebrand, N.; Auffre´dic, J. P.; Loue¨r, D. Chem. Mater. 1998, 10, 2450-2461.

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Table 1. Experimental Preparation of Microemulsions microemulsion

surfactants (ratio wt %) wt %

heptane wt %

FF/TEOS/ NH4OH wt %a

1a 1b 1c 2

AOT 22.5 AOT/Brij30 (50/50 wt %) 22.5 Brij30 22.5 SDS/propanol (67/33 wt %) 40

67.5 67.5 67.5 40

10 10 10 20

a With a percentage weight ratio for FF/TEOS/NH OH of 35/ 4 43/22.

Figure 1. Pseudoternary phase diagram for the surfactant/ heptane/aqueous phase system. For detailed composition of the circles labeled 1 and 2, see Table 1. room temperature under vacuum for 15 min and heated at 800 °C in air for 8 h, while another part was used for the microemulsion process. In the last case, just before use for the microemulsion, the powder was freshly peptized by a new nitric acid treatment (called “double peptization”), washed with acetone, and dispersed by addition of the desired amount of water to obtain stable ferrofluid. 2.3. Synthesis of Zinc Ferrite-Silica Core-Shell Nanoparticles. The silica nanoparticles (with a ZnFe2O4 core) have been prepared by low-temperature W/O microemulsion techniques. Microemulsions are defined as clear thermodynamically stable dispersions of two immiscible liquids containing appropriate amounts of surfactants.32 A W/O microemulsion consists of an oil phase, a water phase, and surfactants and possesses specific physicochemical properties such as transparency, isotropy, and thermodynamic stability.32 n-Heptane was used as the oil phase with different amounts of surfactant (and/or cosurfactant) and aqueous phase (Table 1 and Figure 1). The aqueous phase contained the ferrofluid (FF), TEOS, and NH4OH solution. In more detail, the microemulsion was prepared by adding freshly prepared ferrofluids [0.160-0.35 mol/L range for concentration of ZnFe2O4] into a surfactant/heptane mixture. The produced optical transparency indicated the formation of the W/O microemulsion. Then, TEOS was used to prepare silica nanoparticles by adding it to the microemulsion. After addition of TEOS into a surfactant/heptane/ferrofluids mixture, the silica shell was synthesized by increasing the pH inside the droplets with ammonia (28%) to catalyze the condensation of TEOS.36 TEOS is an organophilic molecule and therefore is more readily dissolved in heptane than in aqueous droplets. Nevertheless, in our case, adapting the work of Arriagada et al.,36 the hydrolysis of TEOS was catalyzed by the acid ferrofluid (pH ≈ 2) initially present in the droplets. Indeed, the Si(OC2H5) group is quickly protonated under such conditions, making alcohol a better leaving group and increasing, therefore, the hydrolysis kinetics. Hydrolysis involved the reaction of the alkoxide with water, generating Si(OH) groups. The hydrolysis reaction may be represented as Si(OC2H5)4 + mH2O f Si(OC2H5)4-m(OH)m + mC2H5OH. The Si(OH) groups generated by the hydrolysis step were used in the based-catalyzed condensation step, inducing the formation of Si-O-Si or Si-OH-Si bonds via olation and oxolation or alcoxolation.32,36 According to ref 36, we supposed that after 48 h, a majority of the monomeric species of TEOS have been hydrolyzed and consequently have migrated into the droplets for condensation and formation of SiO2. In the case of Brij30 surfactants, the reaction temperature should be kept below 25

°C, to avoid any complications due to the presence of a possible thermally induced phase inversion. Although an ultrasonic treatment was used for microemulsion 2 with SDS and 2-propanol as surfactants, it was impossible to keep the sol optically clear on the addition of ammonia. After 48 h, all the samples were washed with n-heptane, ethanol, and acetone removing oil and surfactants, followed by a separation in a centrifuge at 5000 rpm for 15 min. The resulting powder was dried at 60 °C under vacuum. 2.4. X-ray Powder Diffraction. Powder X-ray diffraction data were recorded at room temperature on a Philips PW 3020 using Bragg-Brentano geometry with Cu KR radiation (40 kV, 30 mA) and a secondary monochromator. Data were collected over 10 e 2θ/degrees e 100, in 0.02 steps, with integration times of 8 s. The average apparent crystallite size (β) was evaluated from a whole diffraction pattern profile analysis, using the last development in the Fullprof program (version 2.0, Nov 2001, LLB, Juan Rodriguez-Carvajal). By description of instrumental and intrinsic profiles by the normalized Voigt function (convolution of Gaussian and Lorentzian functions), size and strain effects can be separated from a profile analysis based on the different angular dependence of the Gaussian and Lorentzian full width at half-maximum (HG and HL, respectively) (J. RodriguezCarvajal and T. Roisnel, private communication). “Perfect” Y2O3 powder was used as a standard to determine the instrumental resolution function of our diffractometer. Observed line broadening was modeled by isotropic size effects, leading to 1/cos θ dependent terms of HG and HL (Y and G parameters in Fullprof, respectively) contributions to the size effects. Refinement of tgθdependent isotropic strain parameters (U and X parameters in Fullprof) did not improve significantly the profile fitting. For each diffraction pattern, a counter zero point, the unit-cell parameters were refined in addition to the Y and G parameters. The background level was defined by a polynomial function. β can be related to the “true” size (i.e., in the case of crystalline nanodomains, the size estimated by transmission electron microscopy), but only if the crystallite shape is known or assumed. For instance, for a spherical crystallite, the true diameter D is simply derived from 4/3(β). This simple relation is strictly valid for a monodisperse system.37 2.5. Morphological Investigation by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Scanning electron microscopic photographs by a Hitachi S-5000 were taken to examine the shape and the particle size of silica. A transmission electron microscope (JEOL-2000 FX electron microscope operating at 200 kV) was used to study the shape and size of the zinc ferrite nanoparticles. Samples were prepared by direct deposition of diluted ferrofluid or dry powder dispersed in ethanol on carbon-activated Cu grids. 2.6. Specific Surface Area. The BET (Brunauer, Emmett, and Teller) specific surface area of the samples was determined by following the standard N2-adsorption method, using a Micromeritics ASAP-200 instrument. Samples were degassed at 175 °C for 2 h, and the N2 adsorption isotherms were determined at 77 K. 2.7. Chemical Analysis. For titration of zinc and iron, inductively coupled plasma (ICP) atomic emission spectroscopy was performed using a spectrophotometer from Seiko Instruments Inc., SPS 1700HVR. The resonance wavelengths were λ(Zn) ) 213.86 nm and λ(Fe) ) 259.94 nm. 2.8. Magnetic Measurements. Magnetization versus temperature measurements were carried out with a Quantum Design MPMS2 SQUID magnetometer equipped with the RSO (reciprocating sample option) working in the temperature range 1.8300 K. No correction was made for the diamagnetic contributions of the different cations. In the case of the field-cooled magnetization (FCM) curves resulting from genuine field-cooled measurements, the points have been recorded in cooling the system which differ from points recorded in warming the system after the sample has been cooled to lowest temperature under external field.

3. Results and Discussion 3.1. ZnFe2O4 Nanoparticles. Parts a and b of Figure 2 show the observed X-ray diffraction (XRD) patterns of

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Figure 2. Observed (dots), calculated (full line), and difference X-ray powder diffraction profiles of zinc ferrite nanoparticles before (a) and after (b) annealing in air at 800 °C for 8 h.

the typical freshly dried ferrofluid and the 800 °C annealed samples, respectively. In the case of the freshly dried ferrofluid, the diffraction peaks are broad and similar to those typically observed for nanoparticles. As expected, annealing at higher temperature causes grain growth. In both cases, the d spacing values and relative intensities of the peaks can be unambiguously attributed to the presence of ferrite. The lattice parameters for these two samples have been evaluated by X-ray diffraction and were found to be 8.462(5) and 8.443(1) Å for the freshly dried ferrofluid and the annealed samples, respectively. The average apparent crystallite size (β) of the particles determined by X-ray diffraction was 4.1 and 23.6 nm for the fresh and annealed samples (800 °C), respectively. The observed, calculated, and difference X-ray diffraction profiles are shown in Figure 2. The χ2 factor, which represents the goodness of fit, is equal to 1.1 and 1.4 for the freshly dried ferrofluid and the 800 °C annealed samples, respectively. Figure 3 shows electron micrographs of zinc ferrite nanoparticles taken by TEM. The particles appeared to be polydisperse, and although they were not completely spherical in shape, the average particle diameter was estimated to range between 4 and 6 nm, which was fairly consistent with the β values evaluated by X-ray diffraction. To conclude about XRD and TEM studies, the nanoparticles were assumed to be

Figure 3. TEM image of zinc ferrite. The bar corresponds to 20 nm.

monodomain and free of strain. The chemical analysis of annealed particles revealed that the nanoparticles of zinc ferrite were nonstoichiometric with an average formula Zn0.87Fe2.09X0.04O4 (X represents vacancies). Zinc ferrite obtained by alkalizing nitrate salt aqueous mixtures

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exhibited a large specific surface area (130 m2/g), whereas the value of the specific surface area fell to 12 m2/g after a treatment at 800 °C. Very recently, a preparation of ZnFe2O4 ferrofluid was reported in the literature,35 which is, to our knowledge, the first reported synthesis. In this preparation, after the coprecipitation step and HNO3 treatment, the colloidal particles were boiled with a 0.5 mol L-1 Fe(NO3)3 solution to prevent gradual dissolution of particles or gelation of solution. This treatment is normally necessary to keep the ferrofluid stable for more than 1 day. Nevertheless, in our case the X-ray diffraction of annealed (800 °C) flocculated ferrofluid after nitrate treatment has shown the presence of hematite as an impurity whereas the same dried and annealed (800 °C) ferrofluid without treatment appeared as a single phase (Figure 2). Different nitrate solutions were tested (Zn and Fe nitrate solution (ratio 0.5), pure Zn or pure Fe nitrate (0.5 mol L-1)), and in whole cases, hematite was found as an impurity. This heating of the nanoparticle is very effective to prove the single phase of zinc ferrite. In the previously cited study, the role played by the nitrate treatment in the case of mixed ferrite was interpreted as a shell formation of amorphous ferric hydroxides on the particle surface, which improves the chemical stability. But, the magnetic properties are very much dependent on surface modification of the nanoparticles1 and the presence of impurities. Also contrary to the previous preparations,35 our acidic precipitate was not stirred for 30 min in boiling solution of an aqueous nitrate to prevent gradual dissolution of particles or gelation of solution26,35 but only a double peptization process was applied, which turned out to keep the ferrofluid stable against gel formation for a few months and avoided the presence of impurities. 3.2. Zinc Ferrite-Silica Core-Shell Nanoparticles. Nowadays, a variety of oxide materials have been prepared, as summarized in ref 32, using the microemulsion medium. In one scenario, the metal ions are first solubilized (aqueous pseudophase) in the water pools of the W/O microemulsion. Then the precipitant, in the form of an aqueous solution or gas phase, is introduced in the microemulsion. In a second scenario, the precipitant is first solubilized in the polar core and the metal-containing solution is subsequently added to the microemulsion. But the stability of the microemulsion decreases with an increase in metal concentration, and the feasibility of designed nanoparticle (core-shell) synthesis depends on the availability of a surfactant/oil/water formulation that gives a stable microemulsion before, during, and after precipitation reactions.32 In our process, the possibility to add directly the previously prepared colloidal solution of metal oxide as an aqueous pseudophase in the droplets seems to be one of the solutions to increase the metal concentration without destabilization of the microemulsion before the synthesis of core-shell nanoparticles. The comparison of the XRD powder pattern of zinc ferrite-silica core-shell nanoparticles (microemulsion 1a) with that of zinc ferrite nanoparticles (Figure 4) showed that the broad diffraction peaks were relative to those of amorphous silica for 15 < 2θ < 33. As expected, the XRD peaks assigned to the presence of ferrite were observed with good relative intensities. As already reported after microemulsion synthesis of γ-Fe2O3-silica core-shell nanoparticles,33 the specific surface area of the zinc ferrite-silica nanoparticles was 100 m2/g. After annealing at 800 °C, this material exhibited a specific surface area of 35 m2/g as compared to 12 m2/g for uncoated ferrites. Considering the relationship between the specific surface

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Figure 4. XRD patterns of zinc ferrite-silica core-shell nanoparticles (A) and zinc ferrite nanoparticles (B).

Figure 5. SEM images of zinc ferrite-silica nanoparticles (a, b, c, and d corresponding to microemulsions 1a, 1b, 1c, and 2, respectively). The bar corresponds to 150 nm.

area and the catalytic activity, this phenomenon could improve the catalytic activity of zinc ferrite nanoparticles. The SEM micrographs of the powder are presented in Figure 5a-d. In Figure 5a, the silica particles were shapeless in the microemulsion 1a (see Table 1). Probably, the ability of NaOH to promote the hydrolytic decomposition of an ionic surfactant (such as AOT or SDS) would contribute to the destabilization of the surfactant aggregates.32 On the contrary, as shown in parts b and c of Figure 5 (microemulsions 1b and 1c, respectively), the silica nanoparticles consisted of an arrangement of relatively uniform particles. The diameter of the silica particles in microemulsions 1b and 1c was in the range of 40-60 nm. This result was in very good accordance with the typical size distribution of droplets of water/ heptane microemulsion systems obtained by the dynamic light scattering (DLS) method.32 The system that showed the most uniform particle size distribution and the particles with the most perfect spherical shape was the microemulsion 1b (Figure 5b) using the mixture of AOT and Brij30 as the surfactant phase. This result seems to

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accredit the commonly considered fact that the presence of two kinds of surfactants at the interface between water and oil phases gives flexibility, in addition to lowering the interfacial tension. These two points normally cause easier surface bending to energetically favored dispersion.32 Espiard et al.38 have already shown that for the same conditions (water/surfactant molar ratio and water/ alkoxide molar ratio), the AOT-based microemulsion produced larger silica particles than the poly(oxyethylene)based microemulsion did. Our results also demonstrated the important role played by the nature of the surfactant even if a precise characterization has not been completed. A similar physical state may also arise with SDS as the surfactant. But in the case of the SDS/propanol surfactant and cosurfactant system, the situation is more complicated. One can distinguish in Figure 5d the formation of nascent spherical silica nanoparticles in the size range 30-50 nm but accompanied by an aggregation process. We speculated that interparticle collisions occurred which supports agglomeration, but a precise characterization of the W/O microemulsion is necessary. A recent investigation39 of the structural transition in the microemulsion system of SDS/n-butanol/n-heptane/water has shown that the W/O microemulsion can be formed at low water content. The oil phase is continuous with increasing water/ oil ratio until the phase transition (W/O to oil-in-water (O/W) microemulsions). In this study, the water/surfactant/heptane mixture yields a W/O microemulsion according to the ternary diagram presented in ref 39. Nevertheless, the addition of ammonia changed drastically the ternary diagram, and after addition of ammonia it was impossible to keep the W/O microemulsion stable and therefore the interparticle spacing. The TEM micrographs of samples 1a and 1b are shown in parts a and b of Figure 6, respectively. The presence of ferrite nanoparticles was confirmed by TEM experiments and also corroborated by XRD results. The TEM study also confirmed the shape of the silica particles as observed by SEM results. In the case of microemulsions 1a and 2, it was impossible to synthesize single coreshell nanoparticles. For microemulsion 1a, more than one magnetic core within a silica shell was observed, whereas for microemulsion 2 aggregated empty silica nanoparticles were synthesized. The first point could suggest further particle growth linked to intermicellar matter exchange. The most important feature in Figure 6b is that for microemulsion 1b (the same results were observed for microemulsion 1c), a single nanoparticle of zinc ferrite is located near the center of a spherical silica particle. 3.3. Magnetic Properties. In this section, we will focus on the thermal behavior of the magnetization of the three different systems: (i) aggregated particles (or powders), (ii) colloidal solution, and (iii) zinc ferrite-silica coreshell nanoparticles. In those systems, it is possible to distinguish two main differences, the interparticle distances and the media in which the particles are immersed. The modifications of the distances between particles change the interparticle interactions. If the interactions can be neglected, the magnetic properties are governed by intrinsic properties of the nanoparticles. Above the blocking temperature, the superparamagnetic susceptibility follows the Curie law, χsup ) Csuper/T. In practice, it is not true in the whole temperature range because the intrinsic magnetization Mnr of each particle varies with the temperature, especially when T approaches the Curie

temperature Tc. If the interactions between the particles are activated, MT decreases as T is lowered. In taking into account the thermal dependence of the magnetic moments, MT versus T is flattened above TB. Below TB, the situation is simpler. In the absence of interactions, the FCM increases and reaches a plateau as T approaches a temperature Tsat, while in the limit of strong interactions FCM saturates just below TB. 3.3.1. Aggregated Zinc Ferrite Nanoparticles. The temperature dependence of the ZFCM (zero field cooled magnetization) and FCM curves of an aggregated sample recorded under 10 Oe are presented in Figure 7. The ZFCM increases with increasing temperature, passing through a broad maximum at Tmax ) 32.5 K. On cooling, the FCM coincides with the ZFCM down to 30 K and then the two curves significantly differ as the sample is cooled further. As a result, the FCM also passed through a maximum at Tmax, but much less pronounced than in the zero field cooled (ZFC) mode. Despite the fact that this unusual and unexpected feature has already been reported in numerous nanosystems,3-5,40-43 its physical origin remains obscure.

(38) Espiard, P.; Mark, J. E.; Guyot, A. Polym. Bull. 1990, 24, 173179. (39) Mo, C.; Zhong, M.; Zhong, Q. J. Electroanal. Chem. 2000, 493, 100-107.

(40) Lopez Perez, J. A.; Lopez Quintela, M. A.; Mira, J.; Rivas, J.; Charles, S. W. J. Phys. Chem. B 1997, 101, 8045-8047. (41) Zysler, R. D.; Fiorani, D.; Dormann, J. L.; Testa, A. M. J. Magn. Magn. Mater. 1994, 133, 71-73.

Figure 6. TEM image of zinc ferrite-silica nanoparticles (a and b corresponding to microemulsions 1a and 1b, respectively). The bar corresponds to 20 nm.

Zinc Ferrite Nanoparticles

Figure 7. Magnetization versus temperature curves of aggregated zinc ferrite nanoparticles recorded at 10 Oe in ZFC and FC modes.

Figure 8. Magnetization versus temperature curves of fresh and aged colloidal solutions of zinc ferrite nanoparticles recorded at 10 Oe in ZFC and FC modes. Inset: Zoom of the FCM curve of the fresh colloidal solution in the low-temperature region.

On one hand, Testa et al.42 have recently suggested that the presence of a maximum of MFC in Fe2O3 is the result of the combination of the blocking of the magnetic moment of each particle and the magnetic interactions between the particles. On the other hand, Pileni et al.5 have claimed that in nonstoichiometric zinc ferrite it is due to the intrinsic ferrimagnetic structures of the particles. 3.3.2. Colloidal Solution of Zinc Ferrite Nanoparticles. The ZFCM-FCM versus T curves of a freshly prepared ferrofluid, recorded under 10 Oe, are presented in Figure 8. The magnetic curves are perfectly superimposed regardless of the nanoparticle concentration, which means that interparticle interactions do not operate in our colloidal solution. The curves look very similar to those reported by Pileni et al.5 The ZFCM increases with increasing temperature and passes through a maximum at Tmax ) 18.4 K. On cooling, the FCM superimposes to ZFCM down to 20 K. Then, they break away as the temperature is decreased further producing FCM situated above ZFCM. The FCM passes through a broad maximum (see inset of Figure 8), much less pronounced than in the ZFC mode. Tmax is located at a temperature 2 times lower in our sample than in Pileni’s sample which has a comparable average nanoparticle size (3.7 nm). However, the direct comparison between the blocking temperatures of the two nonstoichiometric zinc ferrite samples synthesized by two different techniques is hazardous: (i) the iron content is different, and (ii) the inversion of the cation distribution may also be different. These two differences (42) Zysler, R. D.; Fiorani, D.; Testa, A. M. J. Magn. Magn. Mater. 2001, 224, 5-11. (43) Ammar, S. Private communication.

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Figure 9. Magnetization versus temperature curves of zinc ferrite-silica core-shell nanoparticles (Brij30) recorded at 10 Oe in ZFC and FC modes.

certainly influence the magnetization amplitude and also the magnetic anisotropy. In Figure 8, we have also included the ZFCM and FCM curves of the same ferrofluid preparation but measured 3 months after in order to look for time evolution (aging process). Tmax has raised to 28 K, while the magnetization at Tmax has diminished. The FCM also passes through a broad maximum at a temperature just below Tmax. In the single-particle case, Tmax ) TB (the blocking temperature) which is directly proportional to the magnetic anisotropy energy constant K and to the volume of the particle V, TB ∝ KV. So the increase of Tmax may be interpreted in terms of particle growth and/or the augmentation of the anisotropy. In the case of a distribution of the size of the particles, in addition to the arguments presented before, a larger standard deviation may also be responsible for the shift of Tmax because Tmax is mainly governed by the “big” particles.44 Regardless of the physical origins of the shift of Tmax, we suspect that the particles are not in chemical equilibrium within the fluid. The simplest process possibly involves the dissolution of the smaller particles that recombine with the bigger ones in increasing their size. Although the nonequilibrium of the nanoparticles could be related to the fact that a nitrate treatment was not used, we do not yet have a clear explanation of this phenomenon. 3.3.3. Zinc Ferrite-Silica Core-Shell Nanoparticles. As a matter of fact, the core-shell nanoparticles do not exhibit such aging over several months and may be considered as chemically stable. The ZFCM-FCM curves of Brij30 recorded under 10 Oe are plotted in Figure 9. Irreversibility develops at a lower temperature than in the case of the uncoated particles, and this holds for all coating methods. The value of Tmax depends slightly on the coating method. The ZFCM curves pass through a maximum at Tmax ) 11, 11.5, 12.5, and 14 K for AOT/ Brij30, AOT, SDS, and Brij30, respectively. The lower the Tmax, the stronger the magnetization in the irreversible region, in both ZFC mode or field-cooled (FC) mode. In the coated samples, the only possible interaction between particles is magnetic dipole-dipole interaction. GarciaOtero et al.28 have recently shown that Tmax increases with the strength of the interactions. Therefore, the origin of the low-temperature shift of Tmax cannot be found in interparticle interactions. It is highly improbable that the coating alters the core of the particles either chemically (variation of the composition) or physically (variation of the cation inversion). Instead, the surface, in direct contact with the external environment, is sensitive to the modi(44) Sappey, R.; Vincent, E.; Hadacek, N.; Chaput, F.; Boilot, J. P. Phys. Rev. B 1997, 56, 14551-14559.

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disordered spin structure at the surface resulting from a major structural disorder. This maximum is eliminated by a surface atomic rearrangement, due to the grafting of silica. Similar behavior, but much less pronounced, has recently been observed on coated and uncoated Fe3O4 nanosized particles.49

Figure 10. MT versus T (measured in the FC mode) for the three samples. The data have been normalized to the value at 300 K.

fications of the habitat of the nanoparticles.45 The structure of the surface depends on the surroundings and more precisely on the chemical bonds between the atoms at the interface, which is known to have a significant influence on the magnetic properties.46,47 The coating with silica improves the structural order at the surface in diminishing the surface magnetic anisotropy. If what we assume is true, the magnetic anisotropy originating from the surface greatly contributes to the global anisotropy in our zinc ferrite nanoparticles because the blocking temperature varies to about 50% according to the environment. One of the striking features of the coated nanoparticles is that the broad maximum on the FCM curve has disappeared compared to the uncoated ones. The FCM increases as the temperature is lowered and tends to saturate in the low-temperature limit as usual for superparamagnetic entities.1 This indicates that the cusp in the FC mode is not correlated with the core’s ferrimagnetism of nonstoichiometric zinc ferrite nanoparticles because it is highly improbable that the grafting alters the core of the particles. In Figure 10, we have represented the MT versus T curves for the three samples in order to detect the presence of interparticle interactions. We immediately see that the Brij30 and the colloidal solution curves are mixed together above 60 K. We have already mentioned that there are no interactions in the colloidal solution because the M versus T curves are independent of the particle concentration. The interactions are therefore also insignificant in Brij30. We notice that the effects of interparticle interactions are clearly seen in the powdered sample: the MT versus T curve is flattened with respect to the two others; these interactions can be dipolar or exchange type because of the direct contact between the particles. We conclude that interparticle interactions cannot explain the presence of a maximum in FCM at low temperature. In disordered magnetic systems such as spinglasses, the FCM passes through a maximum near Tg.48 We might ascribe the FCM cusp as the consequence of the (45) Prodan, D.; Chane´ac, C.; Tronc, E.; Jolivet, J. P.; Cherkaour, R.; Ezzir, A.; Nogues, M.; Dormann, J. L. J. Magn. Magn. Mater. 1999, 203, 63-65. (46) Bødker, F.; Mørup, S.; Linderoth, S. Phys. Rev. Lett. 1994, 72 (2), 282-285. (47) Respaud, M.; Broto, J. M.; Rakoto, H.; Fert, A. R.; Thomas, L.; Barbara, B.; Verelst, M.; Snoeck, E.; Lecante, P.; Mosset, A.; Osuna, J.; Ould Ely, T.; Amiens, C.; Chaudret, B. Phys. Rev. B 1998, 57 (5), 29252935. (48) Mydosh, J. A. Spin Glasses: an Experimental Introduction; Taylor & Francis: London, 1993.

4. Conclusion Nonstoichiometric nanoparticles of zinc ferrite have been prepared using the coprecipitation process. By use of various processes (colloidal dispersion, microemulsion), the environment of the nanoparticles could be controlled and tuned. It was possible to prepare a stable ferrofluid (colloidal dispersion) without classical nitrate treatment. However, the magnetic study reveals that the particles are not in chemical equilibrium within the fluid. We were also capable of preparing spherical silica nanoparticles with a ZnFe2O4 core. They have been fabricated by the low-temperature W/O microemulsion technique. Our study proves that by using a stable colloidal solution as an aqueous pseudophase it is possible to prepare designed oxide nanoparticles. The preliminary results on the magnetic properties of zinc ferrite nanoparticles evidence the primordial role played by the habitat of the particles. It appears that the magnetic properties of our nanoparticles at low temperature are essentially governed by the interface particle-habitat. Indeed, the blocking temperature is very sensitive to the nature of the surroundings of the particles with values varying between 11 and 32.5 K. In addition, the aging of the magnetic zinc ferrite nanoparticles in ferrofluids is established and we must care about the age of a ferrofluid before comparing results. The absence of aging for coated particles is very appealing in order to get long-living magnetic nanoparticles with the same properties. The disappearance of the FCM cusp of the silica-coated particles is probably a direct consequence of the complicated magnetism that may occur on the surface. Any chemical modification of the surface alters the magnetic properties of the system. We have not presented all the magnetic properties of our systems, particularly those of the aggregated ferrite particles. Additional experiments are currently under way and might reveal the presence of a collective magnetic behavior at low temperature due to the interparticle interactions. To conclude, considering our first idea, that is, the control for the design of nanoparticles, the concept of very stable “nanoreactors” or “nanodroplets” is the essential ingredient for the formation of core-shell nanoparticles. These silica nanoparticles with a ferrite core are potentially useful photocatalytic materials. Another interest is the formation of nanostructured materials by using colloidal crystals as a template, which has potential application in advanced catalysis, where the hierarchical porosity combines efficient transport and high surface area. Acknowledgment. The authors thank T. Laude, S. Takenouchi, and T. Wada for great technical assistance. This work has been funded by the Japan Science and Technology Corporation under the Science and Technology Agency-Centre National de la Recherche exchange program. LA020322B (49) Kim, D. K.; Zhang, Y.; Voit, W.; Rao, K. V.; Muhammed, M. J. Magn. Magn. Mater. 2001, 225, 30-36.